The invention relates to a flat semi-finished product including a coated sheet. The sheet is formed of a fluoroplastics substrate containing fillers and the coating is formed of fluoroplastics. The coating is soft and thin in comparison with the sheet and the coating at least partially covers the surface of the sheet. The invention also relates to a process for producing a flat semi-finished fluoroplastics product, in which a polytetrafluoroethylene powder is mixed with a reinforcing phase to yield a powder composition, the powder composition is cold-pressed to yield a hollow cylinder, the hollow cylinder is sintered and at least one sheet is shaved or peeled from the hollow cylinder. The invention additionally relates to a finished component, in particular a flat gasket, made from the semi-finished product.
Flat gaskets of fluoroplastics generally exhibit better impermeability than fiber gaskets bound with other polymers and better impermeability than graphite gaskets. However, a disadvantage of fluoroplastics gaskets is their relatively low compression stability or the very pronounced set or creep tendency of flat gaskets of fluoroplastics. For that reason, in the case of a flat gasket held between two flanges clamped relative to one another, the surface pressure acting thereon diminishes over time. That results in undesirable leakages, in particular at elevated temperatures.
In the past, in order to reduce creep in fluoroplastics gasket materials, fluoroplastics compositions were used which were filled with inorganic substances such as, for example, glass fibers, quartz, silicon carbide or barium sulfate. Although that did make the gasket material more creep stable and compression resistant, the conformability thereof to the flange surfaces, which may be somewhat rough after machining, was reduced, in such a way that pronounced surface leakage channels were able to form between gasket and flange.
In order to counter that problem, multilayer fluoroplastics composites that are known, for example, from U.S. Pat. No. 4,961,891 were developed, with a sheet being formed of filled fluoroplastics substrate. That sheet has a comparatively thick, soft coating of fluoroplastics on each side. Such multilayer composites exhibit a low short-term leakage rate at low to medium surface pressures due to the good deformation capacity of the soft outer coating and the consequently good conformability to surface roughness.
However, it has surprisingly now emerged that such multilayer composites exhibit similarly low creep stability relative to the gaskets of unfilled or slightly filled fluoroplastics mentioned above, when considered over a relatively longer period. The reason therefor is that the soft outer coatings are too thick. As compared to the known setting processes, the relatively thick coating acts as a lubricating film, which causes the gasket to slide to an unexpectedly high degree relative to the flange surface, thus leading to a loss in prestressing. Accordingly, the tightness of such sealing connections diminishes over time.
It is accordingly an object of the invention to provide a flat semi-finished product and a component of the above-mentioned type made therefrom, in particular a flat gasket, which is distinguished, when used in a sealing connection, by a high level of impermeability and at the same time by high creep resistance, as well as a cost-effective process for producing such a semi-finished product, which overcome the hereinafore-mentioned disadvantages of the heretofore-known products and processes of this general type.
With the foregoing and other objects in view there is provided, in accordance with the invention, a flat semi-finished product, comprising a sheet having a surface. The sheet is formed of a fluoroplastics substrate containing fillers and is formed of a material having a Shore-D hardness of at least 65. A coating disposed on the sheet is formed of fluoroplastics. The coating is softer and thinner than the sheet, it at least partially covers the surface of the sheet and it has an average layer thickness of at most 20 xcexcm.
An advantage of the semi-finished product according to the invention, as described in the previous paragraph, is that, due to the relatively thin top coat, form-locking meshing on a microscopic scale is possible between the surfaces of the component (gasket) punched from the sheet and the surface of a structural element prestressed in relation thereto, for example a flange surface, so that flow is prevented. A form-locking connection is one which connects two elements together due to the shape of the elements themselves, as opposed to a force-locking connection, which locks the elements together by force external to the elements. Creep tendencies are effectively countered by the thin soft coating combined with the relatively hard sheet, in such a way that the compression stability of gaskets that are made, for example, from the semi-finished product according to the invention is significantly improved in comparison with the prior art. At 20 xcexcm at most, the average layer thickness lies in the lower range of conventional flange roughness levels, in such a way that, on one hand, leakage channels caused by surface roughness may be sealed even at low prestressing levels and, on the other hand, the advantageous meshing of the surfaces may occur.
In accordance with another feature of the invention, the thin coating is perforated in places, and only part of the surface of the sheet is covered thereby, preferably 50% to approaching 100%. For example, the coating may be screen-printed on in grid-form or lattice-form, in such a way that coated portions of the sheet preferably take the form of lattice lines extending perpendicularly to one another. The lines completely surround uncoated portions of the sheet surface, thereby preventing the occurrence of continuous leakage channels. This has the advantage of providing no hindrance to the uncoated portions of the sheet surface meshing form-lockingly with the associated surfaces. Such direct meshing hinders creep of the pretensioned gasket considerably.
With the objects of the invention in view, there is also provided a process for producing a flat semi-finished fluoroplastics product, which comprises mixing a polytetrafluoroethylene powder with a reinforcing phase to yield a powder composition; cold-pressing the powder composition to yield a hollow cylinder; sintering the hollow cylinder under an inert gas atmosphere, preferably under a nitrogen atmosphere; and shaving at least one sheet from the hollow cylinder. A polytetrafluoroethylene dispersion is applied to at least one surface of the sheet by dipping, spraying, knife application, atomizing or printing, especially screen printing, to provide the sheet with a thin polytetrafluoroethylene dispersion coating having an average layer thickness of at most 20 xcexcm, preferably 2 xcexcm to 8 xcexcm.
It is important that the sheet exhibit a high level of hardness for the semi-finished product and a gasket made therefrom in order to function according to the invention. When producing the semi-finished products according to the process set forth in the previous paragraph, a high level of hardness is achieved in that the hollow cylinder, from which the sheets are produced by shaving or peeling, is sintered under an inert gas atmosphere, preferably under nitrogen. Furthermore, a relatively long sintering period at the final sintering temperature is also favorable for achieving high sheet hardness.
In accordance with another mode of the invention, the thin coating is applied by surface finishing methods, in particular screen printing, in such a way that the sheet has a thin polytetrafluoroethylene dispersion coating with an average layer thickness of at most 20 xcexcm, preferably 2 xcexcm to 8 xcexcm.
In accordance with a concomitant mode of the invention, the average layer thickness of the film-type or grid-type polytetrafluoroethylene dispersion coating is adjusted as a function of the mesh size of the screen used during screen printing. This enables simple, cost-effective manufacture with high repeat accuracy.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a flat semi-finished product, a component made therefrom and a process for the production thereof, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention as well as within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.